Plenty of clues suggest that liquid water once flowed
on Mars --raising hopes that life could have arisen there-- but
the evidence remains inconclusive and sometimes contradictory

January
5, 2001 -- Mars may once have been a very wet place. A host
of clues remain from an earlier era, billions of years ago, hinting
that the Red Planet was host to great rivers, lakes and perhaps
even an ocean. But some of the clues are contradictory -- they
don't all fit together in a coherent whole. Little wonder, then,
that the fate of water on Mars is such a hotly debated topic.

The reason for the intense interest in Martian water is simple:
Without water, there can be no life as we know it. If it has
been 3.5 billion years since liquid water was present on Mars,
the chance of finding life there is remote. But if water is present
on Mars now, however well hidden, life may be holding on in some
protected niche.

Right: Sedimentary rock layers
like these in Mars's Holden Crater suggest that the Red Planet
was once home to ancient lakes. [more information]

Based on what we have observed so far, Mars today is a frozen desert. It's too cold
for liquid water to exist on its surface and too cold to rain.
The planet's atmosphere is also too thin to permit any significant
amount of snowfall.

Even if some internal heat source warmed the planet up enough
for ice to melt, it wouldn't yield liquid water. The Martian
atmosphere is so thin that even if the temperature rose above
freezing the ice would change directly to water vapor.

Signs of Heavy Flooding

Sign up for
our EXPRESS SCIENCE NEWS delivery

But there must have been water, and plenty of it, in
Mars's past. That is evident from the massive
outflow channels that are found, mostly, in the northern
lowlands. The intensity of the floods that carved these channels
was tremendous, perhaps reaching discharge rates as high as 10,000
times that at which the Mississippi, when flooded, pours into
the Gulf of Mexico.

What caused these giant floods? Was it a climate change, perhaps
brought about by a change in Mars's orbit? Or was the planet's
own internal heat responsible? And, whatever mechanism caused
the floods in the first place, where has all that water gone?
Was it absorbed into the ground where it remains today, frozen?
Or did it dissipate into the Martian atmosphere, where it was
subsequently lost to space? No-one knows for certain the answers
to these questions.

Some scientists believe that the catastrophic floods that
carved the outflow channels occurred nearly simultaneously, releasing
such vast quantities of water that they merged into an ocean
that covered the northern lowlands. Tim Parker of NASA's Jet
Propulsion Laboratory first proposed such an idea in 1989. Parker,
examining images taken by the Viking Orbiters, found what he
believed were remnants of two ancient ocean shorelines, which
he called "contacts," one inside the other, in the
Martian north.

Expanding on this notion, in 1991 Vic Baker of the University
of Arizona, suggested that Mars might not be geologically dead
and permanently frozen. Instead, he proposed, Mars might undergo
cycles, or pulses -- first heating up, releasing groundwater
and forming an ocean in the north, then dissipating the ocean
back into the planet's crust and re-freezing.

More
recently, Jim Head and colleagues at Brown University, found
evidence that is consistent with a shoreline that might indeed
have existed at the inner of Parker's two proposed contacts,
contact 2. Head and colleagues examined elevation data gathered
by the Mars Orbiter Laser Altimeter
(MOLA) on board the Mars Global Surveyor
(MGS) and found that the elevation at points along contact 2
were much closer to a straight line than those at contact 1.
They also found that the terrain below this elevation was smoother
than the terrain above it. Both of these findings are consistent
with the former presence there of an ocean.

But the story doesn't end there. Shortly after Head and colleagues
published their findings, Mike Malin and Ken Edgett of Malin
Space Systems used the Mars Orbital Camera
(MOC) aboard MGS, to take a series of high-resolution images
of contact 2 terrain. Their conclusion: there's nothing there.

Right: In this topographic drawing
of Mars, blue indicates the area where an ocean once may have
existed. Credit: NASA Mars Global Surveyor Project; MOLA Team
Rendering by Peter Neivert, Brown University

And the debate continues. Says Mike Carr of the U.S. Geological
Survey, author of the book Water on Mars, "We're
getting all this new data from MGS, and I think a lot of it is
just not understood yet. It's very hard to understand. The whole
business of the oceans, the evidence is so contradictory."

Mysterious Valleys

Mars's small-valley networks, which occur mainly in the southern
highlands, pose another perplexing problem. Scientists who first
studied images of these valleys thought they resembled river
valleys on Earth. So, they reasoned, a similar process, the runoff
of rainwater, must have formed them.

Above: Nirgal Vallis, south of the eastern part of Valles
Marineris, superficially resembles a river-cut valley on Earth.
[more
information]

For Mars to be warm enough to rain, however, it would have
needed a much thicker atmosphere than it has today. And no-one
has come up with a clear-cut explanation for how such an atmosphere
could have formed.

One alternative theory is that a process known as sapping,
or collapse caused by the softening of the soil by groundwater,
created the valleys. Yet another notion is that perhaps glaciers
covered the regions around the valleys, and that glacial meltwater
carved them. As with Mars's other watery mysteries, however,
the question of how the valley networks formed remains unanswered.

And if these vexing problems weren't enough, recent images
from MOC reveal a startling new puzzle. In nearly a dozen different
locations on Mars - all of them far from the equator - there
are signs that water has been seeping out of the walls of valleys
and craters, forming small gullies. Some scientists speculate
that this activity is very recent, perhaps occurring within the
past 10 years; others say 10 million years is more likely.

Yet
many aspects of these seepage gullies defy common sense. "They
sure look like water-worn features," says Mike Carr, "but
they seem to contradict what we know about the stability of water."
They occur not only in the coldest regions on Mars, but on slopes
facing away from the Sun, where the temperature rarely gets above
minus 50 degrees Centigrade. Yet the water appears to be seeping
out from only 100 meters below the surface, a depth at which
scientists previously believed Mars's crust to be frozen solid.
Scientists are busily working to devise an explanation for this
phenomenon.

Above: Martian gullies in Newton Crater. Scientists
hypothesize that liquid water burst out from underground, eroded
the gullies, and pooled at the bottom of this crater as it froze
and evaporated. If so, life-sustaining ice and water might exist
even today below the Martian surface -- water that could potentially
support a human mission to Mars. [more
information]

There is one additional thorn in the side of those who study
water on Mars. No evidence of carbonates has yet been found anywhere
on the planet. Carbonates are minerals that form readily when
liquid water reacts with carbon dioxide in the atmosphere. If
Mars had abundant liquid water in its past, carbonates should
be detectable in the Martian rock record. The Thermal
Emission Spectrometer (TES) instrument aboard MGS
was designed to look for just such a signature. But so far it
has found none. Perhaps other evaporites, such as sulfates (as
detected in Martian meteorites and interpreted from landing site
analyses), are the dominant material of this type on Mars.

What Next?

New
debates will undoubtedly emerge as data from Mars Global Surveyor
is digested. In 2001, NASA will send a new orbiter to Mars, which
will include a higher-resolution spectrometer to search for carbonates.
In 2003, NASA will send two rovers to Mars to hunt for water's
signatures in rocks and soil on the surface. But many questions
about the history of water on Mars are likely to remain unanswered
until samples are returned from the Red Planet for examination
on Earth. Says Carr, "I think the sample return is what
we want, particularly of sediments. And if we could get samples
of things like this back on Earth, I think it would do an awful
lot to help us understand what's going on [on Mars]."

The Science and Technology Directorate at NASA's
Marshall Space Flight Center
sponsors the Science@NASA web sites. The mission of Science@NASA is to
help the public understand how exciting NASA research is and to help
NASA scientists fulfill their outreach responsibilities.